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Abstract:

By a method that includes coking a residue resulting from distillation of
crude oil under reduced pressure and having API gravity of 1 to 5, an
asphaltene content of 10 to 50%, a resin content of 5 to 30%, and a
sulfur content of 1 to 12% to obtain coke, pulverizing the coke to obtain
a carbon powder, and heating the carbon powder at 1000 to 3500 deg C., a
graphite anode active material for use in a lithium secondary battery is
obtained that has, in X-ray powder diffraction, d002 of not smaller
than 0.3354 nm and not greater than 0.337 nm, Lc(004) of smaller than 100
nm, La(110) of not smaller than 100 nm, and a half width of the peak of a
plane (101) at a diffraction angle (2θ) of 44 degrees to 45 degrees
of not smaller than 0.65 degree.

Claims:

1. A graphite anode active material for use in a lithium secondary
battery wherein the graphite anode active material is, according to X-ray
powder diffraction, not smaller than 0.3354 nm and not greater than 0.337
nm in d002, smaller than 100 nm in Lc(004), not smaller than 100 nm
in La(110), and not smaller than 0.65 degree in a half width of the peak
of a plane (101) at a diffraction angle (2.theta.) of 44 degrees to 45
degrees.

2. The graphite anode active material for use in a lithium secondary
battery according to claim 1 wherein a ratio I(100)/I(101) of peak
intensity in X-ray powder diffraction is not lower than 0.7 and not
higher than 1.

3. The graphite anode active material for use in a lithium secondary
battery according to claim 1 wherein a layer has a ratio I(110)/I(004) of
peak intensity of not lower than 0.2 measured by X-ray diffraction, the
layer having density of not lower than 1.5 g/cm3 and not higher than
1.6 g/cm3 formed by applying a mixture of the anode active material
and a binder to copper foil to be subjected to drying and pressure
molding.

4. The graphite anode active material for use in a lithium secondary
battery according to claim 1 wherein a BET specific surface area is not
greater than 5 m2/g and D50 referring to a volume average
particle diameter is not smaller than 3 μm and not greater than 30
μm.

5. The graphite anode active material for use in a lithium secondary
battery according to claim 1 wherein a half width of the peak of a plane
(101) at a diffraction angle (2.theta.) of 44 degrees to 45 degrees in
X-ray powder diffraction is not smaller than 0.65 degree and not greater
than 2 degrees.

6. The graphite anode active material for use in a lithium secondary
battery according to claim 1 wherein the graphite anode active material
is surface-treated.

7. The graphite anode active material for use in a lithium secondary
battery according to claim 1 wherein the graphite anode active material
is surface-treated with pitch having a softening point of 200 to 350 deg
C. and a fixed carbon content of 50 to 80% by mass.

8. The graphite anode active material for use in a lithium secondary
battery according to claim 7 wherein D50 referring to a volume
average particle diameter of the pitch is 1 μm to 10 μm.

9. The graphite anode active material for use in a lithium secondary
battery according to claim 7 wherein the pitch is optically isotropic.

10. A method for producing the graphite anode active material for use in
the lithium secondary battery according to claim 1 comprising: coking a
residue resulting from distillation of crude oil under reduced pressure
and having API gravity of 1 to 5, an asphaltene content of 10 to 50%, a
resin content of 5 to 30%, and a sulfur content of 1 to 12% to obtain
coke, pulverizing the coke to obtain a carbon powder, and heating the
carbon powder at 1000 to 3500 deg C.

11. The method according to claim 10 further comprising performing
surface treatment by mechanofusion or a wet process.

12. The method according to claim 10 wherein a water content of the coke
is not higher than 1.0%.

13. An anode for use in a lithium secondary battery comprising the
graphite anode active material for use in a lithium secondary battery
according to claim 1.

14. The anode for use in a lithium secondary battery according to claim
13 further comprising a vapor grown carbon fiber with a fiber diameter of
not smaller than 5 nm and not larger than 0.2 μm.

15. A lithium secondary battery comprising the anode for use in a lithium
secondary battery according to claim 13.

17. A transportation, a power-generating system, or an electrical or
electronic equipment comprising the lithium secondary battery according
to claim 15.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a graphite anode active material
for use in a lithium secondary battery, an anode for use in a lithium
secondary battery, and a lithium secondary battery. More specifically,
the present invention relates to a graphite anode active material for use
in a lithium secondary battery that maintains excellent charge-discharge
cycle characteristics even when highly packed for increasing capacitance,
an anode for use in a lithium secondary battery comprising the graphite
anode active material for use in a lithium secondary battery, and a
lithium secondary battery comprising the anode for use in a lithium
secondary battery. The lithium secondary battery in one embodiment of the
present invention includes a lithium-ion capacitor.

BACKGROUND ART

[0002] Lithium secondary batteries are largely used as power supplies in
portable devices and the like. Diversification in the functions of
portable devices and the like has led to the growth in power consumption.
This has been prompting demands for an increase in the capacitance and
improvement in the charge-discharge cycle characteristics of lithium
secondary batteries. In a lithium secondary battery, usually, a lithium
salt such as lithium cobaltate serves as a cathode active material and
graphite and/or the like serve as an anode active material.

[0003] Capacitance can be increased by enhancing the electrode packing
density of a carbonaceous material used in an anode. When an electrode
packing density is enhanced using a conventional carbonaceous material,
however, deformation and/or the like of the carbonaceous material may
occur to lead to significant degradation of charge-discharge cycle
characteristics.

[0004] Because of this, studies are conducted to improve a carbonaceous
material itself for use in an anode so as to increase capacitance and
improve charge-discharge cycle characteristics. For example, Patent
Document 1 and Patent Document 2 describe composite graphite having a
particular crystal structure. Patent Document 3 describes combined use of
graphite having a particular crystal structure and a vapor grown carbon
fiber having a particular crystal structure. Patent Document 4 describes
a carbon composite material that is obtained by adhering an organic
compound serving as a polymer raw material to carbonaceous particles such
as graphite particles, polymerizing the organic compound, and then
heating it at 1800 to 3300 deg C.

[0005] Patent Document 1: JP
2007-141677 A

[0006] Patent Document 2: WO 2007/072858 A

[0007] Patent
Document 3: JP 2007-42620 A

[0008] Patent Document 4: JP 2005-158718 A

SUMMARY OF THE INVENTION

Problems to be Resolved by the Invention

[0009] The carbonaceous materials disclosed in these Patent Documents have
improved the capacitance and the charge-discharge cycle characteristics
of a lithium secondary battery. However, as requirements on the
performance of a lithium secondary battery grow year after year, further
improvement is desired in a carbonaceous material for use in an anode for
use in a lithium secondary battery.

[0010] A object of the present invention is to provide a graphite anode
active material for use in a lithium secondary battery that has large
capacitance and maintains excellent charge-discharge cycle
characteristics even when highly packed, an anode for use in a lithium
secondary battery comprising the anode active material, and a lithium
secondary battery comprising the anode.

Means for Solving the Problems

[0011] The present inventors have conducted intensive research to achieve
these objects and, as a result, found a novel graphite anode active
material for use in a lithium secondary battery where the numerical
values of the interplanar spacing, the crystallite size, and the half
width of the diffraction peak, measured by X-ray diffraction, fall within
particular ranges. The present inventors also found that a lithium
secondary battery that comprises the anode active material in the anode
has large capacitance and maintains excellent charge-discharge cycle
characteristics even when highly packed with the anode active material.
The present inventors have conducted further studies based on these
findings and have now completed the present invention.

[0012] Thus, the present invention includes the following embodiments.

[1] a graphite anode active material for use in a lithium secondary
battery, in which the graphite anode active material can be, according to
X-ray powder diffraction,

[0013] not smaller than 0.3354 nm and not greater than 0.337 nm in
d002,

[0014] smaller than 100 nm in Lc(004),

[0015] not smaller than 100 nm in La(110), and

[0016] not smaller than 0.65 degree in a half width of the peak of a plane
(101) at a diffraction angle (2θ) of 44 degrees to 45 degrees.

[2] the graphite anode active material for use in a lithium secondary
battery according to [1] in which the ratio I(100)/I(101) of peak
intensity in X-ray powder diffraction can be not lower than 0.7 and not
higher than 1. [3] the graphite anode active material for use in a
lithium secondary battery according to [1] or [2] in which a layer can
have a ratio I(110)/I(004) of peak intensity of not lower than 0.2
measured by X-ray diffraction, the layer having density of not lower than
1.5 g/cm3 and not higher than 1.6 g/cm3 formed by applying a
mixture of the anode active material and a binder to copper foil to be
subjected to drying and pressure molding. [4] the graphite anode active
material for use in a lithium secondary battery according to any one of
[1] to [3] in which a BET specific surface area can be not greater than 5
m2/g and D50 referring to a volume average particle diameter
can be not smaller than 3 μm and not greater than 30 μm. [5] the
graphite anode active material for use in a lithium secondary battery
according to any one of [1] to [4] in which a half width of the peak of a
plane (101) at a diffraction angle (2θ) of 44 degrees to 45 degrees
in X-ray powder diffraction can be not smaller than 0.65 degree and not
greater than 2 degrees. [6] the graphite anode active material for use in
a lithium secondary battery according to any one of [1] to [5] in which
the graphite anode active material can be surface-treated. [7] the
graphite anode active material for use in a lithium secondary battery
according to any one of [1] to [5] in which the graphite anode active
material can be surface-treated with pitch having a softening point of
200 to 350 deg C. and a fixed carbon content of 50 to 80% by mass. [8]
the graphite anode active material for use in a lithium secondary battery
according to [7] in which the D50 referring to the volume average
particle diameter of the pitch can be 1 μm to 10 μm. [9] the
graphite anode active material for use in a lithium secondary battery
according to [7] in which the pitch can be optically isotropic. [10] a
method for producing the graphite anode active material for use in the
lithium secondary battery according to any one of [1] to [9] which
comprises:

[0017] coking a residue resulting from distillation of crude oil under
reduced pressure and having API gravity of 1 to 5, an asphaltene content
of 10 to 50%, a resin content of 5 to 30%, and a sulfur content of 1 to
12% to obtain coke,

[0018] pulverizing the coke to obtain a carbon powder, and

[0019] heating the carbon powder at 1000 to 3500 deg C.

[11] the method according to [10] which further comprises performing
surface treatment by mechanofusion or a wet process. [12] the method
according to [10] or [11] in which the water content of the coke is not
higher than 1.0%. [13] an anode for use in a lithium secondary battery
which comprises the graphite anode active material for use in a lithium
secondary battery according to any one of [1] to [9]. [14] the anode for
use in a lithium secondary battery according to [13] which further
comprises a vapor grown carbon fiber with a fiber diameter of not smaller
than 5 nm and not larger than 0.2 μm. [15] a lithium secondary battery
which comprises the anode for use in a lithium secondary battery
according to [13] or [14]. [16] the lithium secondary battery according
to [15] which comprises at least one solvent selected from the group
consisting of ethylene carbonate, diethyl carbonate, dimethyl carbonate,
methylethyl carbonate, propylene carbonate, butylene carbonate,
ganma-butyrolactone, and vinylene carbonate. [17] a transportation which
comprises the lithium secondary battery according to [15] or [16]. [18] a
power-generating system which comprises the lithium secondary battery
according to [15] or [16]. [19] an electrical or electronic equipment
which comprises the lithium secondary battery according to [15] or [16].

Advantageous Effect of the Invention

[0020] A lithium secondary battery that comprises the graphite anode
active material for use in a lithium secondary battery of the present
invention in the anode has large capacitance and maintains excellent
charge-discharge cycle characteristics even when highly packed with the
graphite anode active material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 shows the X-ray powder diffraction of a graphite anode
active material for use in a lithium battery in one embodiment of the
present invention that is obtained in Example 1.

[0022] A graphite anode active material for use in a lithium secondary
battery in one embodiment of the present invention, according to X-ray
powder diffraction, has d002 of not smaller than 0.3354 nm and not
greater than 0.337 nm, and preferably not smaller than 0.3359 nm and not
greater than 0.3368 nm. d002 indicates the crystallinity of
graphite.

[0023] d002 is interplanar spacing calculated from the 002
diffraction peak of graphite powder and the Bragg equation d=λ/sin
θc.

[0024] The anode active material in one embodiment of the present
invention, according to X-ray powder diffraction, is smaller than 100 nm,
and preferably not smaller than 40 nm and not greater than 85 nm in
Lc(004). The anode active material in one embodiment of the present
invention, according to X-ray powder diffraction, is not smaller than 100
nm in La(110).

[0025] Lc(004) is the thickness of a crystallite in a c-axis direction
calculated from the 004 diffraction peak of graphite powder. La(110) is
the width of a crystallite in an a-axis direction calculated from the 110
diffraction peak of graphite powder.

[0026] The anode active material in one embodiment of the present
invention, according to X-ray powder diffraction, has B101 referring
to the half width of the peak of a plane (101) at a diffraction angle
(2θ) of 44 degrees to 45 degrees of not smaller than 0.65 degree,
preferably not smaller than 0.65 degree and not greater than 2 degrees,
and more preferably not smaller than 0.7 degree and not greater than 1.5
degrees.

[0027] When the B101 referring to the half width of the peak of a
plane (101) is not smaller than 0.65 degree, the peak is relatively
broad. A broad peak is assumed to indicate that the ABA-stacked structure
of a graphite crystal is disordered. It is known that an ABA-stacked
structure is transformed into an AAA-stacked structure when a lithium ion
is inserted into a graphite layer. When the ABA-stacked structure is
disordered, transformation of the stacked structure of graphite at the
time of lithium ion insertion is speculated to occur at lower energy.

[0028] The anode active material in one embodiment of the present
invention, according to X-ray powder diffraction, has the ratio
I(100)/I(101) of the peak intensity of preferably not higher than 1, more
preferably not lower than 0.7 and not higher than 1, and further
preferably not lower than 0.75 and not higher than 0.95.

[0029] In the anode active material in one embodiment of the present
invention, a mixture layer with density of not lower than 1.5 g/cm3
and not higher than 1.6 g/cm3 formed by applying a mixture of the
anode active material and a binder to copper foil to be subjected to
drying and pressure molding has a ratio I(110)/I(004) of the peak
intensity that is preferably not lower than 0.2 and is more preferably
higher than 0.35 and not higher than 0.9 measured by X-ray diffraction.
The ratio I(110)/I(004) of the peak intensity thus measured indicates the
orientation of graphite powder. The greater the ratio I(110)/I(004) is,
the lower the orientation is.

[0030] The BET specific surface area of the anode active material in one
embodiment of the present invention is preferably not greater than 5
m2/g and is more preferably 1 to 4.5 m2/g. When the BET
specific surface area is not greater than 5 m2/g, an undesirable
side reaction with an electrolyte solution is less prone to proceed, and
deterioration in charge-discharge cycle characteristics is less prone to
proceed.

[0031] The D50 referring to the volume average particle diameter of
the anode active material in one embodiment of the present invention is
preferably not smaller than 3 μm and not greater than 30 μm, is
more preferably not smaller than 4 μm and not greater than 25 μm,
and is further preferably not smaller than 4 μm and not greater than
20 μm. When the D50 referring to the volume average particle
diameter is within the range, the surface of an electrode is smooth and
an undesirable side reaction with an electrolyte solution is less prone
to proceed.

[0032] The anode active material in one embodiment of the present
invention can be obtained, for example, by the following method.

[0033] First, crude oil of Venezuelan origin is distilled under reduced
pressure to obtain a residue. The residue has the API gravity of
preferably 1 to 5, the asphaltene content of preferably 10 to 50%, the
resin content of preferably 5 to 30%, and the sulfur content of
preferably 1 to 12%.

[0034] The residue is coked to obtain coke. A coking method may be delayed
coking or fluid coking. The resulting coke is cut out with water and is
heated, followed by drying to achieve a water content of preferably not
higher than 1.0%.

[0035] The dried coke lump is pulverized and is classified to obtain a
carbon powder. A pulverizing method is not particularly limited, and
examples thereof include a method using an apparatus such as a hammer
mill, a pin mill, a jet mill, a rod mill, and an ACM pulverizer. The
D50 referring to the volume average particle diameter of the carbon
powder after classification is preferably not smaller than 3 μm and
not greater than 30 μm, is more preferably not smaller than 4 μm
and not greater than 25 μm, and is further preferably not smaller than
4 μm and not greater than 20 μm.

[0036] The carbon powder is heated preferably at 1000 to 3500 deg C., more
preferably at 2000 to 3400 deg C., and further preferably at 2500 to 3300
deg C. so as to be converted into graphite. Thus, the anode active
material in one embodiment of the present invention can be obtained.

[0037] The anode active material in one embodiment of the present
invention may be surface-treated. Examples of the surface treatment
include surface fusion by mechanofusion or a similar method, surface
coating by a wet process or a similar method, and the like.

[0038] The wet process is, for example, a method described in JP
2005-158718 A, and is specifically a method comprising adhering an
organic compound serving as a polymer raw material to the surface of the
anode active material and/or impregnating the surface of the anode active
material with an organic compound serving as a polymer raw material,
polymerizing the organic compound, and heating it at 1800 to 3300 deg C.,
or a method comprising adhering a solution of a resin material to the
surface of the anode active material and/or impregnating the surface of
the anode active material with a solution of a resin material, drying,
and heating at 1800 to 3300 deg C.

[0039] Mechanofusion is, for example, a method comprising placing the
anode active material and different species of carbon materials or resin
materials in equipment for fast rotation mixing, applying mechanical
energy to the anode active material and the different species of carbon
materials or resin materials to cause a mechanochemical reaction, and,
where appropriate, performing heating at 900 deg C. to 2000 deg C. In the
present invention, surface treatment with mechanofusion is preferable.

[0040] In the surface treatment of the anode active material, a carbon
material such as petroleum pitch, coal pitch, and coal tar and/or a resin
material such as phenol resins and furan resins is (are) used. Petroleum
pitch and coal pitch are optically isotropic or optically anisotropic. In
examples of the present specification, an optically isotropic one is
used. The pitch used in the surface treatment has the softening point of
preferably 200 to 350 deg C., the fixed carbon content of preferably 50
to 80% by mass, and the D50 referring to the volume average particle
diameter of preferably 1 μm to 10 μm. The amount of the pitch used
in the surface treatment is preferably 0.1 to 50 parts by mass and is
more preferably 0.1 to 10 parts by mass relative to 100 parts by mass of
the anode active material.

[0041] The graphite anode active material for use in a lithium secondary
battery in one embodiment of the present invention may be composed of one
species of carbonaceous material or may be composed of a plurality of
different species of carbonaceous materials, provided that it satisfies
the above characteristic values.

2) Anode for Use in Lithium Secondary Battery

[0042] An anode for use in a lithium secondary battery in one embodiment
of the present invention comprises the anode active material in one
embodiment of the present invention.

[0043] In the anode for use in a lithium secondary battery, the anode
active material is usually in an anode active material layer. The anode
active material layer is formed, by various methods, of a mixture of the
anode active material, a binder, and an additive that is compounded where
appropriate. The anode active material layer usually has a collector
stacked thereon for facilitating energization with a terminal, a
conductive wire, and the like.

[0045] Examples of the additive that is compounded in the anode active
material layer where appropriate include a conductive additive, an
ion-permeable compound, a thickener, a dispersant, a lubricant, active
carbon, and the like.

[0046] Examples of the conductive additive include conductive metal
powders such as a silver powder; powders of conductive carbon such as
furnace black, Ketjenblack, and acetylene black; a carbon nanotube, a
carbon nanofiber, a vapor grown carbon fiber, and the like. The anode in
one embodiment of the present invention preferably comprises a vapor
grown carbon fiber as the additive. The fiber diameter of the vapor grown
carbon fiber is preferably not smaller than 5 nm and not larger than 0.2
μm. The content of the vapor grown carbon fiber is preferably 0.1 to
10% by mass relative to the mass of the anode active material layer.
Examples of the ion-permeable compound include polysaccharides such as
chitin and chitosan, crosslinked products of the polysaccharides, and the
like. Examples of the thickener include carboxymethylcellulose, polyvinyl
alcohol, and the like.

[0047] The anode active material layer is obtained, for example, by
applying the mixture in paste form to the collector, followed by drying
and performing pressure molding, or by pressure molding of the mixture in
granular form on the collector. The thickness of the anode active
material layer is usually not smaller than 0.04 mm and not greater than
0.15 mm. The pressure to be applied at the time of formation can be
adjusted so as to obtain an anode active material layer of any electrode
density. The pressure to be applied at the time of formation is
preferably about 1 t/cm2 to 3 t/cm2.

[0048] Examples of the collector include conductive metal foil, a
conductive metal mesh, perforated plate of a conductive metal, and the
like. The conductive metal used contains copper, aluminum, nickel, and/or
the like. The collector used in the anode preferably contains copper.

3) Lithium Secondary Battery

[0049] A lithium secondary battery in one embodiment of the present
invention comprises the anode for use in a lithium secondary battery in
one embodiment of the present invention. The lithium secondary battery in
one embodiment of the present invention includes a lithium-ion capacitor.

[0050] The lithium secondary battery in one embodiment of the present
invention further comprises a cathode. As the cathode, one that is
conventionally used in a lithium secondary battery can be used. The
cathode usually comprises a cathode active material layer comprising a
cathode active material and a collector stacked on the cathode active
material layer. Examples of the cathode active material include
LiNiO2, LiCoO2, LiMn2O4, and the like. The cathode
active material layer may further comprise a conventionally known cathode
active material additive. The collector used in the cathode preferably
contains aluminum.

[0051] In the lithium secondary battery, the cathode and the anode are
usually immersed in an electrolyte. The electrolyte may be liquid, gel,
or solid.

[0052] Examples of the liquid electrolyte include a solution of a lithium
salt in a nonaqueous solvent. Examples of the lithium salt include
LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3,
CH3SO3Li, CF3SO3Li, and the like. The nonaqueous
solvent in the liquid electrolyte is preferably at least one selected
from the group consisting of ethylene carbonate, diethyl carbonate,
dimethyl carbonate, methylethyl carbonate, propylene carbonate, butylene
carbonate, ganma-butyrolactone, and vinylene carbonate.

[0053] Examples of the solid electrolyte or the gel electrolyte include
polymer electrolytes such as sulfonated styrene-olefin copolymers,
polymer electrolytes comprising polyethylene oxide and MgClO4,
polymer electrolytes having a trimethylene oxide structure, and the like.
The nonaqueous solvent in the polymer electrolytes is preferably at least
one selected from the group consisting of ethylene carbonate, diethyl
carbonate, dimethyl carbonate, methylethyl carbonate, propylene
carbonate, butylene carbonate, ganma-butyrolactone, and vinylene
carbonate.

[0054] A separator is provided, where appropriate, between the cathode and
the anode. Examples of the separator include a nonwoven fabric, a woven
fabric, a microporous film, and the like, and a combination thereof, etc.

[0056] The present invention is described more specifically by examples.
The scope of the present invention is, however, not limited to these
examples.

[0057] The physical properties of a graphite anode active material for use
in a lithium secondary battery were measured by the following methods.

"d002, Lc(004), La(110), I(100)/I(101), and B101"

[0058] The measurement was performed by X-ray powder diffraction.
d002 is interplanar spacing calculated from a 002 diffraction peak
and the Bragg equation d=λ/sin θc. Lc(004) is the thickness
of a crystallite in a c-axis direction calculated using a 004 diffraction
peak. La(110) is the width of a crystallite in an a-axis direction
calculated using a 110 diffraction peak. I(100)/I(101) is the ratio of
the peak intensity of a 100 diffraction peak to the peak intensity of a
101 diffraction peak. B101 is the half width of the 101 diffraction
peak at a diffraction angle (20) of 44 degrees to 45 degrees.

"Orientation, I(110)/I(004)"

[0059] Polyvinylidene fluoride (L#9130; n-methyl-2-pyrrolidone solution)
manufactured by KUREHA CORPORATION was added to an anode active material
by a small amount at a time to achieve a solid content of 5% by mass
while kneading. N-methyl-2-pyrrolidone was added thereto, and the
resultant was kneaded to achieve adequate fluidity. Kneading was
performed at 500 rpm for 5 minutes with an NBK-1, which is a defoaming
kneader manufactured by Nissei Corp., to obtain the mixture in paste
form. The mixture was applied to copper foil using an automatic coater
and a doctor blade with a clearance of 250 μm.

[0060] The copper foil to which the mixture was applied was placed on a
hot plate at about 80 deg C. to get rid of water. Drying was then
performed in a vacuum dryer at 120 deg C. for 6 hours. After drying,
pressure molding was performed with a press machine so as to achieve an
electrode density that is calculated from the mass of the solid content
in the mixture and the dry volume of the coating of not lower than 1.5
g/cm3 and not higher than 1.6 g/cm3, thereby obtaining an
electrode sheet composed of stacked layers of the mixture and the copper
foil. The electrode sheet was cut out into an appropriate size and was
affixed to a glass cell for X-ray diffraction measurement, followed by
X-ray diffraction measurement. The ratio I(110)/I(004) of peak intensity
was then calculated, which indicates the orientation of graphite.

"BET Specific Surface Area, Ssa"

[0061] Specific surface area was calculated from the analysis through the
use of the BET method using nitrogen adsorption.

"Volume Average Particle Diameter, D50"

[0062] Two microspatulafuls of graphite and two drops of a nonionic
surfactant (Triton-X) were added to 50 ml of water, followed by
ultrasonic dispersion for 3 minutes. The dispersion was placed in a laser
diffraction particle size analyzer (Mastersizer) manufactured by Malvern
Instruments Ltd. so as to measure particle size distribution, thereby
determining D50 referring to a volume average particle diameter.

Example 1

Production of Graphite A1

[0063] Crude oil of Venezuelan origin was distilled under reduced pressure
to obtain a residue. The residue had API gravity of 2.3, an asphaltene
content of 25%, a resin content of 15%, and a sulfur content of 6.0%. The
residue was placed in a delayed coker for coking to obtain coke. The
resulting coke was cut out with water and was heated at 120 deg C.,
followed by drying to achieve a water content of not higher than 1.0%.

[0064] The dried coke lump was pulverized with a hammer mill manufactured
by Hosokawa Micron Corporation, followed by air classification by a Turbo
Classifier, TC-15N manufactured by Nisshin Engineering Inc., to obtain a
carbon powder having D50 referring to a volume average particle
diameter of 17 μm.

[0065] The carbon powder was packed into a graphite crucible, followed by
heating in an Acheson furnace at 3200 deg C. to obtain graphite A1. The
physical properties are shown in Table 1. The X-ray powder diffraction of
the graphite A1 is shown in FIG. 1.

Example 2

Production of Graphite B1

[0066] Graphite B1 was obtained in the same manner as in Example 1 except
that air classification was performed to obtain a carbon powder having
D50 referring to a volume average particle diameter of 5 μm,
which was used instead of the carbon powder having D50 referring to
a volume average particle diameter of 17 μm. The physical properties
are shown in Table 1.

Example 3

Production of Graphite A2

[0067] Five parts by mass of optically isotropic petroleum pitch having a
softening point of about 275 deg C., a fixed carbon content of 65% by
mass, and D50 referring to a volume average particle diameter of 5
μm was mixed with 95 parts by mass of the graphite A1. The mixture was
placed in a mechanofusion system manufactured by Hosokawa Micron
Corporation and was subjected to a fast rotation. The resultant was
heated at 1200 deg C. for 1 hour in a nitrogen gas atmosphere. After
cooling, the resultant was passed through a sieve with an aperture of 45
μm to obtain graphite A2. Mechanofusion is a technique of applying a
type of mechanical energy to particles of a plurality of different
materials to cause a mechanochemical reaction so as to create a new
material. The physical properties are shown in Table 1.

Example 4

Production of Graphite B2

[0068] Graphite B2 was obtained in the same manner as in Example 3 except
that the graphite B1 was used instead of the graphite A1. The physical
properties are shown in Table 1.

Comparative Example 1 to Comparative Example 4

[0069] For Comparative Examples, spherical natural graphite (hereinafter,
referred to as graphite C1), mesophase carbon (hereinafter, referred to
as graphite D), and scale-like artificial graphite (hereinafter, referred
to as graphite E), all of which are commercially available products, were
prepared.

[0070] Graphite C2 was obtained in the same manner as in Example 3 except
that the graphite C1 was used instead of the graphite A1. The physical
properties are shown in Table 1.

[0071] Each graphite prepared in Examples 1 to 4 and Comparative Examples
1 to 4 was used as an anode active material.

[0072] A lithium secondary battery was produced using the anode active
material by the following method, followed by measurement of discharge
capacitance retention (%) after 200th charge-discharge cycles. The
results are shown in Table 2.

"Production of Lithium Secondary Battery"

[0073] The following process was carried out in a glove box maintained in
a dry argon gas atmosphere with a dew point of not higher than -80 deg C.

[0074] N-methyl-2-pyrrolidone was added to 95 parts by mass of lithium
cobaltate (C-10, cathode active material manufactured by Nippon Chemical
Industrial Co., Ltd.), 3 parts by mass of a binder (polyvinylidene
fluoride: PVDF), and 5 parts by mass of a conductive material (acetylene
black) to obtain a mixture in slurry form. The mixture was applied to
aluminum foil of 25-μm thick. The aluminum foil to which the mixture
was applied was dried in a vacuum dryer at 120 deg C. for 6 hours. After
drying, pressure molding was performed with a press machine so as to
achieve an electrode density that is calculated from the mass of the
solid content in the mixture and the dry volume of the coating of about
3.5 g/cm3, thereby obtaining a cathode. As an anode, an electrode
sheet fabricated in evaluation of orientation was used.

[0075] In an SUS304 cylindrical container, a spacer, a leaf spring, the
anode, a separator (polypropylene microporous film "Celgard 2400"
manufactured by Celgard Corporation), and the cathode were stacked in
this order. An SUS304 cylindrical top cover was placed thereon. The
container and the top cover were crimped together with a coin cell
crimper to obtain a coin cell for evaluation. Five coin cells were
fabricated for each anode active material to be subjected to an
evaluation test.

"Discharge Capacitance Retention (%) after 200th Cycle"

[0076] The coin cells were subjected to the following charge-discharge
test at constant current and constant voltage.

[0077] The 1st and 2nd cycles were conducted as follows. Charging was
performed at constant current of 0.17 mA/cm2 starting at resting
potential to 4.2 V, and from the point when 4.2 V was reached, charging
was performed at constant voltage of 4.2 V. Charging was then paused when
the current value decreased to 25.4 μA. Discharging was performed at
constant current of 0.17 mA/cm2, followed by cutting off at voltage
of 2.7 V.

[0078] The 3rd and later cycles were conducted as follows.

[0079] Charging was performed at constant current of 0.34 mA/cm2
(equivalent to 0.2 C) starting at resting potential to 4.2 V, and from
the point when 4.2 V was reached, charging was performed at constant
voltage of 4.2 V. Charging was then paused when the current value
decreased to 20 μA. Discharging was performed at constant current of
1.7 mA/cm2 (equivalent to 1.0 C), followed by cutting off at voltage
of 2.7 V.

[0080] The proportion of the discharge capacitance after 200th cycle to
the discharge capacitance after 3rd cycle was determined. This was
performed for the five coin cells, and the average value was determined
to serve as "discharge capacitance retention (%) after 200th cycle".

[0081] As shown in Table 2, the graphite anode active material for use in
a lithium secondary battery in one embodiment of the present invention
was found to give a lithium secondary battery with excellent
charge-discharge cycle characteristics.